This invention relates to a process for the deposition of abrasive materials by an electrostatic technique and to formulations that greatly facilitate such a technique.
In the production of coated abrasives by a process in which an abrasive grain is deposited on an uncured or partially cured binder material the most common deposition technique involves electrostatic deposition in which the grain is projected upwards under the influence of an electrostatic field into contact with the binder. These are usually described as UP (for upward projection) processes. The grain is fed from a hopper to a moving belt which is passed through a deposition location, defined by a charged plate located below the moving belt and directly opposite and parallel to a grounded plate located above the moving belt. The substrate on to which the grain is to be deposited follows a path parallel to and above the moving belt as they both pass through the deposition location. The electrostatic field between the charged plate and the grounded plate causes the grain to be projected upwards towards the down-facing surface of the substrate where it adheres to an uncured or partially cured binder coated thereon. Providing the particle size is uniform this usually results in a very uniform deposition of the grain. However if the grain has a tendency to form clumps or if the flow to the surface from which it is projected is uneven, the uniformity of the deposition can be seriously impaired. This problem is particularly serious when very fine particle sizes are involved.
The present invention provides a process for the electrostatic deposition of abrasive particles, even when their size is extremely small. The invention can be used in the feed mechanisms for an UP abrasive grain deposition process or it can be used to deposit a functional powder comprising abrasive grain on the surface of a formulation comprising abrasive grain dispersed within a curable binder in a process such as is described for example in U.S. Pat. No. 5,833,724.
The present invention provides a process for the production of a coated abrasive in which abrasive particles with a grit size smaller than 320 grit are electrostatically deposited on a substrate which comprises treating the abrasive particles prior to deposition with silica powder in an amount sufficient to raise the volume resistivity and the surface resistivity of the abrasive particles by at least fifty percent but to not more than a surface resistivity of 1014 ohms/square and/or a volume resistivity of 1014 ohms.cm. Preferably these maximum resistivity values are less than 1012 ohms/square and ohms.cm respectively.
The surface and volume resistivities are measured using ASTM D4496 which is the standard test method for measuring the xe2x80x9cDC Resistance of Conductance of Moderately Conductive Materialsxe2x80x9d and ASTM D2557 which is the standard test method for measuring the xe2x80x9cDC Resistance of Insulating Materialsxe2x80x9d. Achieving an acceptable level according to the invention in one of the parameters, (volume and surface resistivities), will imply that an acceptable level has also been attained in the other such that measurement of either parameter alone is sufficient in practical terms.
It is found that resistivity values can becontrolled to ensure that the treated powder is more readily adapted to UP deposition in coated abrasive applications. However resistivities that are too low or too high are both undesirable. It is therefore necessary to control the resistivity to secure optimum results. Addition of a silica powder is effective to increase the resistivity of the abrasive particles but too large a resistivity creates projectability problems. A salient characteristic of the powder formulations of the invention in which this is achieved is that they are electrostatically projectable while retaining enhanced flowability.
The desired resistivity values can be obtained for example by adding to the abrasive particles a suitable silica powder additive the amount of which will vary with the additive. In general however it is possible to secure target resistivity properties for the powders of the invention by the addition of from 0.02 to 5% by weight based on the weight of the formulation. The preferred amount of silica is from 0.05 to 3%, such as from 0.1 to 2%, based on the formulation weight.
The silica powder preferably has a particle size no greater than that of the abrasive particles.
The silica can be any of the available powdered silica products such as fumed or precipitated silicas. While silica is inherently somewhat resistant to charge-driven clumping, some silicas such as fumed silica have highly porous particle structures leading to exaggerated surface areas and with such silicas a tendency to form clumps is sometimes encountered. Where such problems are encountered with fumed silica, it can be used effectively after treatment with an additive such as hexamethyldisilazane to increase the hydrophobicity of the silica surface and minimize the tendency to agglomerate. Such treatment is frequently used by commercial suppliers of fumed silica. Even if some agglomeration of commercial fumed silica powder does occur, the forces involved are much attenuated and can readily be broken down by shear stress.
Suitable silicas which can be used with advantage include:
FG-SP FLOW-GARD(copyright) with particle size of 25 microns and a BET surface area of 220 m2/gm;
FG-AB(copyright) with particle size of 20 microns and a BET surface area of 130 m2/gm;
HI-SIL(copyright) T-600 with particle size of 2.0 microns and a BET surface area of 170 m2/gm; and
HI-SIL(copyright) T-152 with particle size of 1.4 micron and a BET surface area of 150 m2/gm; (all these are available from PPG Corporation); and
CAB-O-SIL(copyright) TS-530 which has a particle size of 0.2 micron, a surface area of 220 m2/gm and has been given a surface treatment of hexamethyldisilazane. This product is available from Cabot Corporation.
The abrasive particles can be for example fused or sintered alumina, silicon carbide, cubic boron nitride, diamond or fused alumina/zirconia. The most commonly used abrasives are however based on alumina or silicon carbide. The abrasive particle size that can be used corresponds to 320 grit or finer but the problem is usually encountered in greatest severity at grit sizes of P1200 and finer. This corresponds to average particle sizes of about 25 microns and finer.
The formulation can also comprise, in addition to the abrasive particles and silica powder, functional additives that convey specific properties to the abrasive product such as surface lubrication, anti-static properties, enhanced grinding capabilities and so on. Such additives are included along with and in intimate mixture with the abrasive particles. These too preferably have particle sizes equal to or smaller than the abrasive particle with which they are mixed. The amount of functional additive that can be present can be for example from 5 to 75%, and preferably from 25 to 60% and most preferably from 30 to 50% of the total weight of abrasive plus additive.
Besides having resistivity levels consistent with the invention it is also found that the abrasive powders of the invention are in general much less susceptible to variations in moisture in the atmosphere or on the grain. With some grains, notably alumina-based grains, the relative humidity surrounding the UP deposition apparatus very significantly affects the efficiency by which the abrasive particles are projected. The abrasive particle powders of the invention are however much more resistant to humidity variations, thereby providing a significant extra benefit from the practice of the invention.
When referring to the abrasive particles the size expressed in terms of a CAMI grading process defines an average particle size which corresponds to a specific number of microns. When referring to silica or other powdered additives the particle size is expressed in microns and refers to a volume average particle size as determined by, for example, a Horiba particle size analyzer.